A method for generating electricity comprising the steps:(A) passing a concentrated ionic solution through a first pathway in a reverse electrodialysis unit comprising a membrane stack having electrodes and alternating cation and anion exchange membranes; and(B) passing a dilute ionic solution through a second pathway in said reverse electrodialysis unit, whereby solute from the concentrated solution in the first pathway passes through the membranes to the dilute solution in the second pathway, thereby generating electricity; wherein the concentration of solute in the dilute ionic solution as it enters the reverse electrodialysis unit is at least 0.03 mol/l.